SAR (Synthetic-Aperture Radar) is a microwave pulse-transmission radar system carried by an aircraft.
The use of microwaves enables acquisition of images at any moment, irrespective of whether it is day or night and irrespective of the meteorological conditions, and consequently SAR is widely used in environmental monitoring, in military applications, in geological mapping, and in other applications.
The energy of the radar pulse, transmitted by an antenna, is reflected by ground surface in all directions, and part thereof is captured by the same transmitting antenna. This signal, generally referred to as “echo”, is then processed for generating a sequence of images. The sequence of images is obtained by integrating the radar signal over a given time interval, referred to also as time of observation.
The image resolution in the direction of pointing of the antenna (range direction) depends exclusively upon the bandwidth of the microwave pulses transmitted. The image resolution in the direction perpendicular to the direction of pointing of the antenna (azimuth direction or cross-range direction) depends upon the time of observation, i.e., the time of reception of the echo signal. In order to obtain a high resolution image, quite long observation times are necessary, which generally depend upon the distance observed and upon the velocity of the radar platform, and may even be of the order of tens of seconds. Consequently, recognition and location of possible moving objects in the scene observed are very imprecise. In a way similar to what occurs with an ordinary camera, in fact, a moving object gives rise to a “wake” in the image acquired. Obviously, the wake is all the more evident, the longer the time of observation and the faster the object is moving.